Alistair Darling said the abuse 'is going to get more unpleasant'. Photograph: David Cheskin/PA

Alistair Darling, the former chancellor who is leading the campaign against Scottish independence, has spoken of his anger at a "disgraceful" internet campaign by nationalists against those who speak up for the union.

The Labour MP said Scotland's first minister, Alex Salmond, could stop the "vile abuse" being meted out by the so-called "cyber-nats" to those in favour of Scotland staying part of Britain, but had chosen not to do so.

In an interview with the Observer, Darling also warned that the referendum campaign was becoming increasingly toxic as the date of the vote, 18 September, approached. "It's going to get more unpleasant. You can see it already. You're dealing with something that people have worked towards all their lives, and the stakes will get higher and higher.

"On the first day of 2014, the CBI general secretary in Scotland put out a press release that questioned various of the SNP's assertions. He was subjected to vile abuse from the nationalists. They went after him," said Darling.

Darling, who is MP for Edinburgh South West, said a pattern of abuse had been established and it was up to Salmond to stop it. The first minister has also refused to criticise the treatment of Nigel Farage, Ukip leader, who was barricaded in an Edinburgh pub by a baying mob last year and had to be rescued in a police riot van.

Darling, who said he hasn't made a decision yet about whether he will stand as an MP in 2015, said: "The cyber-nat activity is disgraceful.

"They will trash anyone who disagrees with them. Their intention is to make people keep their heads down. Salmond could stop it, but he doesn't choose to."

In a wide-ranging interview, Darling added that, nine months ahead of the vote, he believed the nationalists were losing the argument by their own criteria. "A few years ago they were saying privately – well, not that privately, as they were telling a succession of journalists – that they had to be at 40% by now."

The latest YouGov poll, commissioned by the unionists, suggests that fewer than a third of Scots (30%) want to leave the UK.Darling also defended the "no" campaign against those who have criticised its failure to capture voters' imaginations by suggesting that he was aware of the risk of the electorate becoming bored so far ahead of the vote. "People ask: why haven't you had rallies and razzmatazz? But one of the problems for both sides is exhaustion on the part of the electorate. Frankly, people can only take three or four weeks of campaigning, then they want you to go away," he said.

Darling, who said the "yes" campaign was seeking to move the debate away from the economic arguments about independence, added that he planned to capture people's "heads and hearts" by emphasising the strengths of the union and its history. "The argument is that you're not [properly] Scottish unless you're with them. It's an offensive argument, but one they're peddling day in, day out."

The former chancellor also revealed that he had dined with the Labour leader, Ed Miliband, shortly before Christmas but denied rumours that he was planning a return to the party's front benches. "One of Ed's many strengths is that, if I've told him once, he doesn't need to ask me again."